TRANSPORTATION RESEARCH RECORD 1089 11 The Pressuremeter in Geotechnical Practice ERNEST WINTER The pressuremeter, Introduced In its present form In the 1950s, led to a new ASTM standard to be introduced in the near has gained substantial acceptance In the United States. Typical future. testing procedures have for the most part not changed since The pressuremeter test consists of the expansion of a mem­ the meter's Introduction, but changes are being recommended brane, usually in a predrilled borehole (Figure 1). The volume by researchers to accommodate test evaluations in clay soils. The test procedure ls In the process of being standardized. change and the pressure are measured in the test, and the Typical uses of the test for foundation design, settlement anal­ pressures to increase the volume are generally applied in pre­ ysis, and pile behavior are reviewed. determined steps. A modulus is then determined to reflect the relation between volume change and pressure, and the pressure at which failure of the soil occurs is also evaluated. It is these The pressuremeter as an in situ testing instrument is well two parameters, the pressuremeter modulus and limit pressure, known and used in the United States and is widely accepted for that are used in evaluations, together with the curve of pressure use in routine investigations. Since its introduction, consider­ versus volume. able research and development have occurred and new testing methods as well as new methods of evaluation have been suggested. It is accordingly considered appropriate to trace this development from its beginning for a better overall understand­ ing of the test of today. ORIGINS OF PRESSUREMETER TEST The modem use of the pressuremeter is based on the efforts of / Louis Menard, a French engineer, who developed the original concept by Kogler into a usable test instrument in the late 1950s. Menard also recognized, however, that new empirical methods will be required to analyze foundation performance GUARD CELL with the pressuremeter and made considerable full-scale mea­ surements to support his empirical correlations. PROBE flEASURING CELL At the introduction of the pressuremeter in the United States, GUARD CELL the instrument was furnished with a whole set of new rules based on Menard's measurements and evaluations. These eval­ FIGURE 1 Schematic section of uation methods were mostly unknown to the engineering com­ pressuremeter. munity. The question was whether those rules would be accepted for the design or whether additional research would be necessary to produce parameters for the Menard rules for The performance of the test as well as methods of evaluation local conditions and on correlation of the test results with will be discussed in some detail in this paper; in addition some known geotechnical parameters. The first category of efforts particularly well-suited and well-developed applications for the was generally pursued by consulting engineers, whereas cor­ engineering design will be reviewed. The subject of this paper relations with shear parameters and compressibility were is the pressuremeter as an instrument in the everyday use of developed mostly by researchers. geotechnical engineering. With this in mind, the single most After more than 20 years of use, a great number of test important factor affecting the quality of the tests, namely, the results are available and typical test values in particular preparation of the borehole, is examined in some detail. geologic formations are known. The results of disturbance during installation are relatively well established, and experi­ ence has shown that the forming of the borehole has a signifi­ HOLE PREPARATION cant effect on the regular pressuremeter test. Efforts to stan­ dardize the procedures of installation as well as the test have Two types of disturbance occur when a borehole is drilled for the test. The first type is the result of opening the hole itself. As the borehole is drilled, a stress release occurs when material is Schnabel Engineering Associates, 4909 Cordell Avenue, Bethesda, removed from the boring. This stress release is restored during Md. 20814. the test or reduced by performing the test as soon as possible 12 TRANSPORTATION RESEARCH RECORD 1089 after drilling. The effect of this unload-load cycle will, expansion of the probe to the minimum. Generally a hole however, exist as long as predrilling is part of the testing. An diameter between 3 to 20 percent larger than that of the probe improvement is the self-boring pressuremeter, which is inserted should be maintained. while the original in situ stresses are maintained. This is still considered a research tool or special testing equipment and is used mostly on larger, elaborate projects in which the higher THE PRESSUREMETER TEST cost of such testing can be absorbed. The second type of disturbance is caused by the drilling The test itself consists of expanding a probe in a predrilled equipment along the walls of the borehole and can be consider­ borehole and measuring volumes and corresponding pressures. ably reduced by the use of the right tools and techniques. In The most commonly used method is to increase pressures in everyday practice, some methods have been developed that are about 10 increments to failure. It has also been acceptable to more useful in one soil than in others. Techniques vary by run the test by controlling strain and measuring the correspond­ geography and country of origin. A tabulation has been ing stress. The principle of the test is, however, to perform the assembled by ASTM for the upcoming standard, showing test in a relatively short period of time, thereby measuring methods suited for particular subsoil conditions (Table I). As basically undrained conditions, even where pore-pressure dis­ can be readily seen, the most widely applicable method is hand sipation is significant. Because the test results are sensitive to augering, which causes probably the least disturbance in most the speed of testing, specifically in fine-grained soils, this soils. The soils representing the largest problems in hole prepa­ makes standardization more important. It has been the practice ration are the sands and gravels, especially below ground water of the engineering community to perform tests according to the level. In these soils good results were obtained, however, by original recommendations of Menard in order to maintain the using mud to keep the boring open. comparative value of the results. Another factor, also related to preparation of boreholes and As part of the testing procedure, the drill hole is advanced to disturbance, is the range of tolerances to be maintained to the test level and cleaned of debris and cuttings. Before the obtain a good test. The pressuremeter generally requires a hole probe is placed in the hole, all calibrations are completed and in which only minimal space is maintained between the probe checked. The probe is then lowered to the test depth and and the walls of the borehole in order to keep the necessary pressures are applied in predetermined steps. The load incre- TADLE 1 GUIDELINES FOR SELECTION OF BOREHOLE PREPARATION METHODS AND TOOLS Sandy Sandy Gravel Carey Silty Loose or Gravely Preparation Firm- Stiff- Above Below Above Below Medium- Sands a Weathered Method Soft Stiff Hard GWL GWL GWL GWL Dense Loose Dense Rock Rotary drilling with bottom discharge of prepared mud l lb 2 NR Pushed thin wall sampler 2 2b NR NR NR NR NA NA NA Pilot-hole drilling With subsequent sampler pushing 2 2 2 NR NR NR NR NA NA With simultaneous shaving 2 2 2b 2 2 2 NA NA NA Continuous flight auger NR lb NR 2 NR NA NR Hand auger In the dry NR NA NR 2 NR NA NA NA With bottom discharge of prepared mud NA 2 NA NA NA Driven or vibro­ driven sampler NR NR NA 2 NR 2 NR 2 NR NR Core barrel drilling NR NR 2b NR NR NR NR 2 2 2 Rotary percussion NR NR lb NR NR NA NA NA NA 2 Driven, vibro-driven, or pushed slotted tube NR NR NR NR NR NR NR NR 2 le NR Note: GWL = gl'Oundwatcr level; NR = not recommended; NA = not applicable; 1 = !mt choice; 2 = second choice. "Below GWL. bMethod applicable only under certain conditions (see text). cPilot-hole drilling required beforehand. WINTER 13 ments are estimated from testing in similar material and from cmRECTED \Q..IME READING ( V) experience, in order to complete the test in about 10 load increments. Readings are taken after 30 sec and 1 min during TEST each load step. The test is complete when the probe expansion URVE becomes excessive. The test may include cyclic loading to evaluate load-unload characteristics. The volume-pressure curve is plotted with all necessary corrections. The test yields the pressuremeter modulus, which is the slope of the pressure-volume curve, and the limit pressure, ' '1 /"-..._CREEP which is the pressure at which unlimited expansion occurs. For : , ,/ CURVE evaluation in practice, the pressuremeter modulus is generally determined from the straight-line portion of the pressuremeter Pe PRESSLRE APPLIED TO curve and the limit pressure is taken at a volume expansion that BOREHOLE WALL ( P) is twice the original probe volume. For all applications in FIGURE 2 Typical pressuremeter general practice, these determinations are adequate and will be test curve. discussed in more detail later. It is noted here that several pressuremeters are in use today, including the most widely accepted original Menard type, in the volume readings and pressure increments used to determine which three cells are used within the expandable probe. The the pressuremeter modulus. Harder or denser soils will result in center measuring cell is protected by two guard cells, one at the a flatter curve and higher pressuremeter modulus, whereas the upper and one at the lower end, to prevent excessive expansion plastic deformation represented by the right end of the curve of the measuring cell in those directions.